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Tuesday, August 20, 2013

Eavesdropping on a foraging pilot whale

The
world of a deep-diving toothed whale is hard to imagine. However, it is
even more challenging to imagine how the whales themselves perceive
their underwater environment.

Frants Jensen analysing data from arecently tagged pilot whale

Toothed whales depend on echolocation sounds for
orientation and foraging. They send out high-amplitude, highly
directional biosonar signals but have to listen for the tiny fraction
of sound energy reflected off prey and other objects. Foraging requires
both detecting and discriminating the faint echoes from prey through
background noise but also closing in on and capturing prey up close.

The beauty of echolocation-based foraging is that we can use acoustic recording tags to eavesdrop on the foraging of tagged animals such as sperm whales or beaked whales, effectively tapping into their own biosonar system.

A time-frequency representation (spectrogram)of a foraging buzz produced during prey capture

Like
other toothed whales, pilot whales switch from relatively slow clicking
when searching and approaching prey, to periods of very fast clicking (a
foraging buzz) when they get close to prey. These periods of rapid
buzzing can be used for identifying when or where tagged animals forage. This information can also be combined with the other sensors of the tag to identify specific
kinematic maneuvers during prey capture, or determine whether prey was
captured.

Listen in on a pilot whale while it searches for and captures prey

Here, we have taken a dive profile and marked down the different foraging events using some of the auditing software developed for these DTAGs. Five different sequences (labeled A to E) can be listened through by pressing the corresponding button:

C: A handful of foraging buzzes occur near the bottom of the dive, some of them quite long, possibly indicating prey chases.C: Play sound

D: On its way back to the surface, the pilot whale is mostly silent, but occasionally calls out for group members.D: Play soundE: As the tag breaks the surface of the water, the animal catches its first breath of fresh air in 14 minutes.E: Play sound